1. Field of the Invention
The present invention relates to a coating method suitable for forming a plurality of layers of coating on a flexible support running at a high speed in order to manufacture a magnetic recording medium, a photographic sensitive material, an electronic material, a coating-type battery, an optical film for anti-reflection or the like, abrasive tape, information recording paper, etc.
2. Description of the Prior Art
A technique for simultaneously forming a plurality of layers of thin coating films on a flexible support consisting of a plastic material or the like at a high speed by use of an extrusion-type coating apparatus is disclosed in Japanese Patent No. 2,581,975, JP-A-5-212337 and so on.
However, when the thickness of a coating film which was an undermost layer was made 10 xcexcm or less in a wet state by the technique disclosed in these publications, the air (entrained air) moving with the running support might invade the coating film so as to cause coating unevenness. In addition, when an edge surface of a coating head and the flexible support were brought into a closer relationship to each other in order to form a thin coating film by the technique disclosed in the above-mentioned publications, foreign matters or the like adhering to the surface of the support might be mixed into a coating composition, or the support might be shaved by the edge surface of the coating head so that shavings produced thus were mixed into the coating composition. As a result, there might arise coating failure such as coating streaks or the like.
On the other hand, a technique in which an undercoating composition having low viscosity mainly composed of a solvent is applied onto a support in the upstream of a coating head, and the support is coated with a coating composition while a surplus of the undercoating composition is scraped by an end portion of an edge surface of the coating head is disclosed in Japanese Patent No. 2,601,367 or JP-A-6-134380. Here, a coating composition is applied onto the flexible support in which a gap between the support and the edge surface is sealed with the undercoating composition, so that the entrained air is prevented from invading the coating film. It is therefore possible to form a thin coating film on a support at a high speed.
However, when a coating film was to be formed on a support by the technique disclosed in Japanese Patent No. 2,601,367 or JP-A-6-134380, a solvent of an undercoating composition might pass through an adjacent coating film and make the surface of the coating film rough when the coating film was dried. When a solvent superior in compatibility with a coating composition for adjacent coating films was selected for an undercoating composition, the undercoating composition and the coating composition were dried simultanuously. As a result, such a disadvantage was improved. However, such an advantage might cause a problem in material requiring high degree of surface smoothness, such as a high-density magnetic recording medium or the like.
It is therefore an object of the present invention to solve the foregoing problems to thereby provide a coating method in which two or more layers of thin coating films can be formed on a flexible support at a high speed without making the surfaces of the coating films rough when the coating films are dried.
In order to achieve the foregoing object, according to a first aspect of the present invention, there is provided a coating method in which an extrusion-type coating head having a plurality of doctor edge surfaces is pressed onto a flexible support laid and running between pass rollers so as to coat the flexible support with a coating composition ejected from a slit, the method comprising the steps of: preparing a lower-layer coating composition so as to have a static viscosity of not less than 1 P and a viscosity of not more than 50 cP at a shear rate of 10,000 secxe2x88x921; excessively applying the lower-layer coating composition to a surface of the support; scraping a surplus of the lower-layer coating composition by the coating head so that a lower layer is formed; and applying an upper-layer coating composition, which is ejected from the slit, onto the lower layer.
Preferably, according to a second aspect of the present invention, in the above coating method, respective coating compositions are prepared so that a difference in viscosity at a shear rate of 10,000 secxe2x88x921 between coating compositions for layers adjacent to each other does not exceed 10 cP.
Preferably, according to a third aspect of the present invention, in the above coating method, at least a part of a downstream one of the doctor edge surfaces of the coating head projects over a tangent drawn from a downstream end of a most upstream one of the doctor edge surfaces toward a circumferential surface of a pass roller located just downstream from the coating head, the circumferential surface being in contact with the support.
Preferably, according to a fourth aspect of the present invention, in the above coating method, each of the doctor edge surfaces in the coating head has a curved surface which is convex toward the support, and a minimum value of a curvature radius of the curved surface in any doctor edge surface is smaller than that in any other doctor edge surface located in more downstream side.
The term xe2x80x9cstatic viscosityxe2x80x9d herein means viscosity measured by a Brookfield viscometer. The term xe2x80x9cshear ratexe2x80x9d means a ratio of change of speed of fluid in a direction perpendicular to the flow direction of the fluid flowing with shearing force applied thereto. The shear rate is also referred to as xe2x80x9crate of flowxe2x80x9d.
In the configuration of the coating method according to the first aspect of the present invention, not a background-art undercoating composition but a lower-layer coating composition adjusted in viscosity is excessively applied onto a support at the upstream of a coating head, and a surplus of the lower-layer coating composition is scraped by the coating head so that a lower layer is formed while an upper-layer coating composition is applied onto the lower layer. As a result, it is possible to reduce the interaction of particles in the coating compositions or the flow generated by the distribution of surface tension when a coating film is dried. It is therefore possible to conspicuously restrain the coating film surface from being coarse. The disorder of the interface between the upper and lower layers is also restrained.
Then, if the static viscosity is smaller than 1 P, the coating composition becomes easy to flow in the coating film surface when the coating composition is dried, so that it is impossible to restrain the coating film surface and interface from being coarse. On the other hand, if the viscosity of the lower-layer coating composition at a share rate of 10,000 secxe2x88x921 exceeds 50 cP, the lower-layer coating composition cannot be scraped uniformly by a doctor edge, so that superior coating cannot be attained.
In the configuration of the above-mentioned method according to the first aspect of the invention, a plurality of upper layers may be formed on the lower layer. In this case, any interface between the upper layers adjacent to each other can be also restrained from being disordered.
In addition, with the configurations according to the above-mentioned second to fourth aspects of the invention, it is possible to further conspicuously restrain the coarsening of the coating film surface and the disorder of the interface. The most upstream-side doctor edge surface of the coating head stated in the above-mentioned configuration according to the third aspect of the invention is also referred to as a front edge surface.